In “electronic paper” displays, an active matrix backplane based on Thin Film Transistors (TFTs) is used to address the display front-plane that contains the active display medium, which changes its optical properties in response to the application of an electric field (e.g. in liquid crystalline (LCD) or electrophoretic (EPD) displays), or the passage of electric current (e.g. in electrochromic displays, ECD).
Electrophoretic displays (EPDs) are electric field-driven, i.e. switching requires relatively large voltages (approx. 10V), but only very low current levels. An EPD based on a low-temperature polycrystalline silicon active matrix backplane has recently been presented [see SID International Symposium, Seminar, and Exhibition 2005, Boston, USA; paper 54.3: “A Flexible 2-in. QVGA LTPS-TFT Electrophoretic Display”; H. Kawai, M. Miyasaka, A. Miyazaki, T. Kodaira, S. Inoue, and T. Shimoda of Seiko-Epson Corp., Nagano, Japan, and K. Amundson, R. J. Paolini, Jr., M. D. McCreary, and T. H. Whitesides of E-Ink Corp., MA, USA].
An alternative technology for realizing active matrix backplanes is Organic Field Effect Transistor (OFET) technology, which is based on polymeric, oligomeric, or small molecular semiconducting organic materials. The charge carrier mobility in such organic semiconductors is several orders of magnitude lower than in high-performance inorganic semiconductors such as silicon, which results in correspondingly lower source-drain current levels. Furthermore, due to the larger thickness and lower dielectric constant of the polymeric dielectric layers in OFETs, the gate voltages required for turning the transistor on are higher as compared to silicon TFTs with thin SiO2 dielectric layers. However, the performance of OFETs matches the requirements for driving electrophoretic displays in terms of driving voltage and current levels. Furthermore, an important advantage of OFETs as compared to silicon TFTs is that they can be produced using cost-effective printing technologies (pad, screen, or inkjet printing) for the additive patterning of device components. An example is the inkjet printing of semiconducting polymers to form a transistor channel between source and drain electrodes, and the inkjet printing of silver colloid to form gate electrode lines in top-gate polymer OFETs.
OFETs work in an accumulation mode, i.e. the charge carriers accumulate in the transistor channel in response to the gate voltage applied in the ON-state. In the OFF-state, when no gate voltage is applied, the residual charge carrier concentration (due to unintentional doping of the semiconductor) should be as low as possible in order to avoid cross-talk between neighboring pixels, and in order to maximize the bi-stability of the display pixel switching states.
In contrast to EPDs, electrochromic displays (ECDs) operate at very low driving voltages (0-2V), but require relatively high current levels for switching the pixel coloration states. An active matrix ECD demonstrator comprising inkjet-printed TiO2-particle pixel electrodes, a viologen-based electrochrome, and a polycrystalline silicon TFT back-plane has recently been presented [see SID International Symposium, Seminar and Exhibition 2006, San Francisco, USA; paper 4.5: “The Design and Driving of Active-Matrix Electrochromic Displays Driven by LTPS TFTs”, S. W-B. Tam, B. McGregor, and M. Ishida of Cambridge Research Laboratory of Epson, Cambridge, U.K., H. Kawai, S. Nebashi, and T. Shimoda of Seiko-Epson Corp., Nagano, Japan, and D. Corr, U. Bach, N. Leyland, F. Pichot, and P. Brien of NTERA, Ltd., Dublin, Ireland].
A major obstacle in using OFETs as switching transistors for ECDs is the low current output of OFETs. This results in prohibitively long switching times for updating the information content in ECDs based on OFETs.
As discussed above, the low current levels in OFETs are due to the low charge carrier mobility in most organic semiconductors, combined with a relatively low charge carrier concentration in the transistor ON-state. The latter is partly due to the large thickness and low dielectric constants of the dielectric polymer layers commonly used in OFETS. These factors lead to the specific capacitance of the transistor channel in OFETs being small in comparison to silicon TFTs, which also results in, high gate voltages of the order of several tens of Volts being required for turning the transistor channel on. Lowering the gate voltage by reducing the thickness of the dielectric layer in an OFET is difficult because a thinner dielectric polymer layer results in increased leakage currents and the risk of a dielectric breakdown. Furthermore, whilst increasing the dielectric constant of the dielectric polymer results in an increased gate capacitance, it generally decreases the charge carrier mobility at the semiconductor-dielectric interface [see “Low-k Insulators as the Choice of Dielectrics in Organic Field-Effect Transistors”, J. Veres, S. D. Ogier, S. W. Leeming, D. C. Cupertino, S. Mohialdin Khaffaf, Adv. Funct. Mat. 13, 199 (2003)].
Whereas the low current levels provided by OFETs are presently not sufficient to drive ECD pixels, an alternative technology based on electrochemical transistors (ECTs) has been demonstrated to provide high current levels. In such ECTs, the transistor channel is bridged by an electrochemically active, conducting polymer such as p-doped PEDOT-PSS. Adjacent to the conducting polymer layer, an electronically insulating but ionically conducting electrolyte layer in contact with a counter electrode (“gate electrode”) allows an electric field to be applied to the electrochemically active material in the transistor channel (the “working electrode”, in terms of electrochemistry), thus changing its oxidation state. When the conducting polymer is reduced from its p-doped (conducting) form to its neutral (semiconducting) form, the conductivity drops drastically.
ECTs based on conducting polymers, which are stable in their p-doped state, operate in a depletion mode. Prior to application of the gate voltage, the transistor is in an electrochemically stabilized ON-state, where negatively charged counterions are present in the bulk of the conjugated polymer, balancing the positive charges on the polymer chains, and thereby allowing for much higher charge carrier densities as compared to conventional OFETs in their ON-state. Due to this increased charge carrier density, ECTs can deliver much higher current levels than OFETs. Application of a positive gate voltage relative to the source contact switches the transistor channel from its initially p-doped ON-state to its electrochemically reduced OFF-state [see US2006202289, WO03047009, WO02071505 and SE0100748].
For p-doped PEDOT-PSS, conductivities in excess of 500 S/cm can be achieved [see H. Elschner of H. C. Starck GmbH, Germany, “High conductive PEDOT/PSS—a polymeric alternative to inorganic TCO”; presented at the “XIIIth International Seminar: Commercial Applications of Conductive Polymers”, Oct. 9-11, 2006, at the Villa La Pietra, Florence, Italy], while the conductivity drops to below 0.01 S/cm upon reduction to the neutral state of PEDOT [see J. R. Reynolds, and M. C. Morvant, Synth. Met. 92, 57 (1998)]. For ECTs based on PEDOT-PSS and operating with highly resistive electrolytes, ON/OFF ratios exceeding 105 have been reported [see D. Nilsson, M. Chen, T. Kugler, T. Remonen, M. Armgarth, and M. Berggren, Adv. Mater. 14, 51 (2002)].
Another variant of ECTs is based on organic semiconductors that are stable in their neutral, non-doped state, i.e. the same organic semiconductor materials as are commonly used in OFETs [see T. Masateru and K. Tomoji, “Vertical electrochemical transistor based on poly(3-hexylthiophene) and cyanoethylpullulan”, Appl. Phys. Lett. 85, 3298 (2004); S. Chao and M. S. Wrighton, “Solid State Microelectrochemistry: Electrical Characteristics of a Solid State Microelectrochemical Transistor Based on Poly(3-methylthiophene)”, J. Am. Chem. Soc. 109, 6627 (1987); and T. G. Bäcklund, H. G. O, Sandberg, R. Österbacka, and H. Stubb, “Current modulation of a hygroscopic insulator organic field-effect transistor”, Appl. Phys. Lett. 85, 3887 (2004)].
Furthermore, an electrochemical transistor based on a hydrogen-terminated diamond layer as an inorganic semiconductor which is gated by a water-based liquid electrolyte has been described in U.S. Pat. No. 6,833,059.
As in ECTs based on conducting polymers, the neutral organic semiconductor in the transistor channel is gated electrochemically with an electrolyte comprising mobile ions. However, these devices work in an accumulative mode, i.e. prior to the application of the gate voltage, the transistor is in its non-doped OFF-state. Applying a negative voltage to the gate (counter) electrode then results in electrochemical doping of the transistor channel and switching to the ON-state.
Both variants of ECTs have the benefit of operating at very small gate voltages (0V to −2V). The electrochemical doping and de-doping processes of the material in the transistor channel are driven by the potential applied between the gate (counter) electrode and the polymer in the transistor channel (working electrode). In contrast to OFETs, where the electric field extends throughout the dielectric layer between the transistor channel and the gate electrode, the electric field in electrolyte-gated ECTs is confined to the electrolytic double layer capacitances formed at the interfaces between the electrolyte and the transistor channel and gate electrode, respectively. As the specific capacitance of electrolytic double layers (μF/cm2) is far larger than the specific capacitance in conventional OFETs (nF/cm2), the gate voltages required for switching ECTs are drastically reduced as compared to OFETs [see M. J. Panzer, C. R. Newman, and C. D. Frisbie, “Low-voltage operation of a pentacene field-effect transistor with a polymer electrolyte gate dielectric”, Appl. Phys. Lett. 86, 103503 (2005)].
Furthermore, the amplitude of the conductivity change in ECTs is independent of the distance between the gate electrode and the transistor channel, i.e. the thickness of the electrolyte layer. This allows lateral transistor configurations to be used, where the gate electrode is patterned from the same PEDOT-PSS layer as the source and drain contacts, at a distance of millimeters from a conjugated polymer channel [see D. Nilsson, M. Chen, T. Kugler, T. Remonen, M. Armgarth, and M. Berggren, “Bi-stable and Dynamic Current Modulation in Electrochemical Organic Transistors”, Adv. Mater. 14, 51 (2002)].
The doping state within a conjugated polymer in the transistor channel of an ECT depends on its electrical potential relative to the potential of the surrounding electrolyte (which is controlled by the gate counter electrode). When a voltage is applied between the source and drain contacts, the difference between the electrical potential within the conjugated polymer and the surrounding electrolyte changes along the transistor channel. Consequently, the degree of doping within the transistor channel becomes a function of the position relative to the source and drain electrodes. This variation of the doping degree as a function of position results in a concomitant change of the local conductivity within the transistor channel, and the occurrence of current saturation (pinch-off) at low source-drain voltages (i.e. when the magnitude of the source-drain voltage |VDS| less than the magnitude of the gate voltage |VG|) in the output characteristics of ECTs [see N. D. Robinson, P.-O, Svensson, D. Nilsson, and M. Berggren, “On the Current Saturation Observed in Electrochemical Polymer Transistors”, J. Electrochem. Soc. 153 H39 (2006)].
The use of electrolytes for addressing the transistor channel in ECTs, replacing the dielectric layer of OFETs, solves the problems of the low charge carrier concentrations and current levels of OFETs. Instead of dropping over the entire thickness of the dielectric layer, the potential difference between the gate electrode and the semiconductor material in the transistor channel of an ECT drops within the electrolytic double layer capacitances that are formed at the interfaces between the electrolyte and the gate electrode, and the electrolyte and the semiconductor in the transistor channel. As the thickness of these electrolytic double layer capacitances is far less than the thickness of the dielectric polymer layers commonly used in OFETs, the electric field strength within the electrolytic double layers, and hence the specific capacitance of the electrolytic double layers, are far larger than the electric field and the gate capacitance in conventional OFETs (of the order of μF/cm2 instead of nF/cm2). Correspondingly, the charge carrier concentration for a given gate voltage is drastically increased by the use of ECTs, and the gate voltages required for switching ECTs are drastically reduced as compared to OFETs.
Furthermore, the influx of counterions from the electrolyte into the bulk of the semiconductor in the transistor channel of an ECT during electrochemical doping stabilizes the charge carriers in the semiconductor material and thereby allows for even higher charge carrier concentrations.
However, a problem arises in the use of ECTs due to the electrochemical degradation of organic semiconductors in common electrolytes. The accumulation of holes (i.e. the formation of reactive carbo-cations, in chemical terminology) in the semiconductor material of an ECT at highly positive potentials often results in chemical reactions with species in the electrolyte, resulting in the disruption of the conjugated system and the irreversible loss of electrochemical activity. An example is the electrochemical degradation of polyaniline, which may occur very rapidly in aqueous electrolytes, due to nucleophilic attacks that result in the hydrolysis of the polymer chain.
Likewise, polythiophene films are easily “over-oxidised” in water-containing electrolytes, with both the degree of “over-oxidation” and the potential at which it occurs being highly dependent on the amount of water present in the electrolyte. The mechanism proposed to explain the “over-oxidation” of polythiophenes involves the oxidation of the sulphur atoms in the thiophene rings, followed by elimination of SO2 [see “Anodic overoxidation of polythiophenes in wet acetonitrile electrolytes”, Barsch, U. and F. Beck, Electrochimica Acta 41, 1761 (1996)].
In addition, the stability of the electrolyte itself is a major problem for electrochemical transistors (ECTs). Electrolytes need to have a constant, high ionic conductivity, a large electrochemical window within which the electrolyte is neither reduced nor oxidized at the cathode or anode, fast ion mobility during the redox intercalation/de-intercalation reactions, low volatility, and environmental stability.
Whilst proton-conductors allow for high ionic conductivities (e.g. Nafion displays 0.1 S/cm), acidic electrolytes are easily reduced at the cathode and result in the formation of hydrogen. Another issue with both water-based and organic solvent-based electrolytes is that they will eventually dry out and therefore require hermetic encapsulation to ensure a long device lifetime. The problem of solvent evaporation can be circumvented by using polymer-based solid electrolytes (such as Li salts dissolved in polyethylene oxide (PEO)). However, such solid electrolytes display comparatively low ionic conductivities.